Documentation for an extension

Hi, I'm new to the list, so I hope this is the right place to ask my question, if not, my excuses! Currently I'm working on a PHP extension for Ceph ( https://github.com/ceph/phprados/<https://github.com/ceph/phprados/tree/oo-interface>).It's not yet ready to be adopted by PECL. I would like to create already some documentation on it's API, preferably in such a way that if one day it's moving upstream a could reuse the existing documentation. I've already did some research about which tools could be used, but it's still a bit unclear to me what's currently the best way to go. Any advice is welcome! Thanks! Jeroen